It depends. Some consider them canon if they fit seamlessly into the established Samurai Jack universe and don't contradict the main storylines. Others might have different opinions.
A great Samurai Jack and Ashi love fanfiction would likely focus on the emotional journey of their relationship. For example, it might start with Ashi being sent to kill Jack but instead finding herself drawn to his honor and kindness. Their love could be a symbol of redemption for Ashi, as she breaks free from her dark past. And for Jack, it could be a new beginning, a chance to have a connection in the desolate world he has been wandering in for so long.
The appeal lies in the contrast between the two characters. Samurai Jack is a paragon of virtue, while Ashi was initially a tool of evil. Their love in fanfiction represents the power of change and redemption. It's exciting to see how they interact and how their relationship evolves from hostility to love.
Perhaps a pairing between Jack and a robotic being. The contrast between his organic, samurai - like nature and the cold, mechanical nature of the robot could lead to a really interesting dynamic in the fanfiction. They could learn from each other's strengths and weaknesses.
To be honest, I don't know for sure. It depends on the storyline and the creators' decisions. You could look for reviews or discussions online to find out.